This invention relates generally to traction motor dynamic braking systems in locomotives and more particularly to an air-cooled resistor grid package for a dynamic braking system.
In a conventional rail locomotive, a diesel engine is used to drive an alternator. The alternator, in turn, supplies electrical current to drive a plurality of electrical traction motors. The traction motors provide the motive force for propelling the locomotive in the forward and reverse directions. In addition to providing a driving force, the traction motors may also perform a braking function. In the braking mode, the traction motors are configured to generate electricity instead of consuming it. As generators, the traction motors convert the kinetic energy of motion of the locomotive into electrical energy, thereby providing a dynamic braking action to slow the movement of the locomotive. The electrical energy generated during dynamic braking can not be used or stored conveniently on-board the locomotive, so it is converted to heat energy by connecting the traction motors to a bank of electrical resistors. Such electrical resistors are commonly called dynamic braking grids. The dynamic braking grids are cooled by fan-driven air, thereby transferring the energy generated by the dynamic braking to the ambient environment.
A typical stack of braking grids may occupying a volume of only about 50 cubic feet and may be used to dissipate approximately 1.8 MW of power. A limiting factor in the amount of dynamic braking force that may be applied to a locomotive is the upper temperature limit of the materials of the dynamic braking grids. The efficient transfer of heat energy from the resistors to the ambient environment is critical to the proper performance of a dynamic braking system. Because the design of the braking grid package is subject to size and noise limitations, it is not always possible to simply increase the number of braking resistors and the size and capacity of the cooling fans.
Working within predetermined design boundaries, it is desirable to minimize hot spots in the braking grids in order to maximize the energy dissipation across the entire grid while avoiding localized material failure. A typical fan will provide a very uneven airflow velocity distribution at the fan outlet, as illustrated in FIG. 1. Typically, the outlet velocity is highest proximate the center of the impeller fan blades 10 and lowest at the root and tips of the blades. Therefore, it is known in the art to provide a flow diffuser plate between the fan outlet and the resistor stack inlet. The flow diffuser plate is a flat plate 12 typically formed of metal and having a pattern of holes 14 formed there through, as illustrated in FIG. 2. In the annular ring area 16 of the plate 12 aligned with the high velocity portions of the fan airflow, the quantity and/or size of holes 14 per unit area of the plate is relatively low. In the central area 18 and corner areas 20 of the plate 12 aligned with the low velocity portions of the fan airflow, the quantity and/or size of holes 14 per unit area of the plate is relatively high. This uneven distribution of openings in the diffuser plate 12 has the effect of making the distribution of airflow volume and velocity downstream of the diffuser plate 12 much more even than that provided at the fan outlet, as illustrated in FIG. 1. The diffuser plate 12 also serves to reshape the air stream from the generally circular cross-section of the fan blades 10 to the generally rectangular cross-section of the downstream resistor grid stack 22. Thus, the cooling provided across the resistor grid stack 22 is more evenly distributed as a result of the action of the diffuser plate 12 and hot spots therein are minimized or eliminated.
Unfortunately, the prior art diffuser plate 12 is essentially a flow blocking device and it creates a significant pressure drop in the air stream, thereby reducing the total volume of cooling airflow provided through the resistor grid stack 22. To compensate for this airflow reduction, a larger and/or more powerful fan motor 2 may be provided, with the associated cost, weight and noise penalties.
Thus, there is a need for an improved locomotive dynamic braking grid package. In particular, there is a need for an air delivery system for a resistor grid stack that provides a high volume flow of air having a relatively constant cross-sectional velocity profile.
An apparatus for at least partially normalizing an axial flow velocity distribution of a flow of cooling air supplied by a fan to a locomotive dynamic braking grid resistor stack is described herein as including: a flow turning vane disposed in the flow of cooling air downstream of the fan and upstream of the resistor stack, the flow turning vane oriented within the flow of cooling air to direct a portion of the cooling air from a relatively higher velocity portion of the flow of cooling air into a relatively lower velocity portion of the flow of cooling air. The flow turning vane may include an annular member having an inside diameter dimension that decreases along an axis in the direction of the airflow for directing a portion of the cooling air from a relatively higher velocity annular portion of the flow of cooling air into a relatively lower velocity center portion of the flow of cooling air. The flow turning vane may further include a corner member disposed proximate a corner of a duct bounding the flow of cooling air for directing a portion of air from a relatively higher velocity annular portion of the flow of cooling air into a relatively lower velocity corner portion of the flow of cooling air. The apparatus may include a first flow turning vane and a second flow turning vane disposed in the flow of cooling air downstream of the first flow turning vane and upstream of the resistor stack.
A cooling apparatus for a locomotive dynamic brake resistor grid stack is described herein as including: a fan for inducing a flow of air having a cross-section with a relatively higher velocity area and a relatively lower velocity area; a duct for directing the flow of air away from the fan to an inlet to a dynamic brake resistor grid stack; and a flow directing diffuser disposed within the duct for directing a portion of the flow of air from the relatively higher velocity area into the relatively lower velocity area to at least partially normalize a flow velocity distribution of the air entering the inlet to the grid stack. The fan may be a mixed flow fan.
A locomotive dynamic braking grid package is described as including: a plurality of electrical resistors packaged in a grid stack; a fan for producing a flow of cooling air; a duct for directing the flow of cooling air from the fan to the grid stack for cooling the plurality of electrical resistors; and a flow turning vane disposed within the duct for directing a portion of the cooling air from a higher axial velocity area into a lower axial velocity area of the duct to at least partially normalize an axial flow velocity profile of the cooling air as it enters the grid stack. The fan may be a mixed flow fan.
In a further embodiment, a locomotive dynamic braking grid package is described as including: a plurality of electrical resistors packaged in a grid stack; a mixed flow fan for producing a flow of cooling air; and a duct for directing the flow of cooling air from the fan to the grid stack for cooling the plurality of electrical resistors. The locomotive dynamic braking grid package may further include an annular flow turning vane disposed within the duct for directing a portion of the cooling air from a higher axial velocity annular area into a lower axial velocity center area of the duct to at least partially normalize an axial flow velocity profile of the cooling air as it enters the grid stack.